International Concrete Abstracts Portal

The strength and toughness durability of carbon fiber reinforced cements (CFRC) in inorganic acidic environments was investigated by subjected prismatic flexural specimens (15 x 15 x 150 mm) to two acidic environments (H2 and HNO4) at the age of 28 days for up to 90 days. The pH of the two acids was maintained at 4.0. Eight CFRC mixes and three volume fractions of pitch-based carbon fibers were investigated. It was concluded that while plain unreinforced cements had considerable retrogression in their mechanical properties, carbon fiber reinforced cements had no appreciable effect on either the strength or the toughness, at least for the duration of exposure investigated.

Low-calcium fly ash (ASTM Class F) is being increasingly incorporated into portland cement concrete as a partial replacement for cement. The replacements commonly used are 15 to 25 percent by weight of cement. CANMET has recently developed concrete in which high volumes of low-calcium fly ash are incorporated with slumps in excess of 150 mm being obtained by the use of large dosages of superplasticizers. Typically, in high-volume fly ash concrete, cement content is kept at about 150 kg/m3 and the water-to-cementitious materiaes ratio is about 0.32. The fly ash content is about 56 per cent by weight of the total cementitious material. This paper presents data on several aspects of durability of this new type of concrete. The aspects discussed include freezing and thawing cycling, resistance to chloride ion diffusion, deicing salt scaling resistance, carbonation, and volume stability. Data on the role of high volumes of fly ash to control alkali-silica reaction in concrete are also presented. It is concluded that, in general, high-volume fly ash concrete has excellent durability characteristics. The only exception is the deicing salt scaling tests, in which the above concrete performs poorly.

Concretes having strengths ranging from 54 to 73 MPA and densities ranging from 1920 to 2080 kg/m3 were produced from two lightweight aggregate sources. Supplementary cementitious materials (including silica fume, fly ash, and ground granulated blast furnace slag) were used in the mixtures. Test specimens were subjected to a variety of freezing and thawing test procedures and conditioning methods. These included standard ASTM test procedures as well as procedures designed to simulate service in arctic offshore environments. The high-strength lightweight concretes exhibited outstanding performance, with virtually no degradation during standard freezing and thawing testing. Prolonged exposure was needed to cause significant damage under simulated arctic offshore conditions. Durability was found to be a strong function of cumulative freezing and thawing cycles and moisture content, with saturation of aggregates prior to test leading to premature failure.

Roller-compacted concrete (RCC) is a non-air-entrained concrete of no-slump consistency, which is placed by spreading in horizontal layers and consolidated by smooth-drum, vibrating rollers. Though used in other applications for many years, RCC in its present form has been used to construct concrete gravity dams by taking advantage of the high production rates and attendant cost savings associated with the use of earthwork equipment to transport, place, and "compact" the material. RCC is finding increasing use in thick paving applications, taking advantage of the high production rates of asphalt paving equipment. The earlier attempts to entrain air in lean, dry RCC mixtures were nsuccessful, and the freeze-thaw (FT) durability of RCC was considered poor. Dam structures were designed so that the RCC was protected from weathering by zones of conventional air-entrained concrete, or "sacrificial" RCC was placed beyond the design cross section. This raised the cost of these structures due to the higher unit cost of the conventional concrete or the added costs associate with the larger cross section. With the expanding use of RCC, particularly in paving applications, it is desirable to attempt to develop air entrainment to satisfy FT durability criteria and allow RCC to fully complete with conventional concrete and asphalt in the marketplace. The Bureau of Reclamation, Denver, Colorado, has been investigating the FT durability of RCC through laboratory testing, outdoor exposure testing and recent field demonstrations. The goal of the program is to reduce the need for conventional concrete facing and expand the applications of RCC where FT durability is a requirement, such as in the facing of embankment dams. This paper summarizes the results of testing performed to date in the laboratory and in the field.

A comprehensive research program was carried out to evaluate the quality of the outer layer of concrete (the covercrete), in terms of its durability and protective value to the underlying reinforcing bars. Part of the results of that investigation are presented in this paper, namely those dealing with the influence of the concrete grade and cement type on the permeability to oxygen and water absorption properties of the 'covercrete' of concrete specimens subjected to 7 days moist + 21 days dry curing. The results obtained show that the permeability to oxygen is reduced 40 times when the concrete strength is raised from 25 to 85 MPa, whilst the rate of water absorption is reduced by a factor of 4. In addition, the cement type (including Portland and blended cements) seems to play only a secondary role in the quality of the 'covercrete' of concretes of the same grade, especially above 25 MPa.